I don't have a lot to say about the basic argument, other than you should be deeply skeptical of anyone who tries to prop up obviously ideological contentions as empirical straw men in order to discredit the political opposition. Mann and Ornstein seem to avoid specifics for the most part, though here's how Froomkin summarizes what they're saying about the GOP:

Lies from Republicans generally and standardbearer Mitt Romney in particular weren't limited to the occasional TV ads, either; the party's most central campaign principles -- that federal spending doesn't create jobs, that reducing taxes on the rich could create jobs and lower the deficit -- willfully disregarded the truth.

These are awfully reductive and debatable statements—if this is how the GOP is willfully disregarding the truth, than the current Democratic insistence that a slight increase in marginal tax rates for the rich is a serious solution to address our exploding debt is problematic by the same standards.

But what I really find astounding is that Ornstein and Mann decided to weigh in on media fact checking. Mann and Ornstein's reputations rest on their vocation as policy experts, and their conclusions expose them as both ignorant and ideological. I actually agree with their conclusion "the fact-checkers may have made things worse rather than better," and that it's no substitute for thorough reporting. But they quickly run off the rails:

And then there was this terrible irony: "Fact checkers almost seemed obliged to show some balance in their fact checking."

"There was some damn good stuff done, and stuff that really did hold Romney to account," Ornstein said. But no fact-checker intent on "appearing to be utterly straightforward, independent, and without an axe to grind, is going to actually do the job of saying that we're going to cover 20 fact checks on one side, to three on the other."

So, Ornstein concluded: "If you looked at where the scales should have been, and where they were, they were weighted. And they weren't weighted for ideological bias. They were weighted to avoid being charged with ideological bias."

Here's the problem: we actually can quantify fact checking pretty easily. And contra Ornstein, the results are that fact checkers are pretty willing to say Republicans are the problem, credibility be damned. I've noted this many times before, but the University of Minnesota Humphrey School of Public Affairs did a study on PolitiFact, the largest media fact checking organization, from January 2010 through January 2011. The study found "current and former Republican officeholders have been assigned substantially harsher grades by the news organization than their Democratic counterparts. In total, 74 of the 98 statements by political figures judged 'false' or 'pants on fire' over the last 13 months were given to Republicans, or 76 percent, compared to just 22 statements for Democrats (22 percent)."

More recently, the Center for Media and Public Affairs at George Mason University tallied up 98 PolitiFact findings from June 1 to September 11:

Conversely, statements by Republicans were rated as entirely false about twice as often as Democratic statements – 29% false ratings for GOP statements vs. 15% false ratings for Democrats. (This includes categories labeled “false” and “pants on fire.”)

The same pattern holds for statements made directly by the presidential candidates and their campaigns. A majority of the Obama campaign’s statements (55%) were rated as true or mostly true, compared to one out of four statements (26%) by the Romney campaign.

The difference is even greater at the other end of the spectrum, where 26% of the Romney campaign’s statements were rated as either false or “pants on fire,” compared to only 5% of the Obama campaign’s statements.

I realize that this isn't as often as Ornstein's preferred 20 to three ratio, but it seems that PolitiFact is willing to come out and call Republicans liars over Democrats at a rate of three to one or two to one. Is Ornstein really suggesting that this isn't enough? Or is he just entirely ignorant of the fact that fact checkers are already pretty skewed? I'm going to go with the latter, because even the editor of PolitiFact seems ignorant of how tilted toward Democrats his own organization's conclusions are. (I'm not even going to get into what a demonstrably terrible job fact checkers did evaluating many of the specific claims made during this last campaign.) Ornstein's real problem seems to be fact checkers weren't as effective as he would like them to be at discrediting Republicans.

It gets better:

Ornstein is particularly infuriated that none of the supposed reader advocates at major newspapers have raised the issues they brought up. "What the f--k is an ombudsman doing if he's not writing about this?" he asked.

As it happens, I was interviewed about fact checking and "false balance" in the media by an ombudsman back in January. Perhaps Ornstein occasionally reads the New York Times? Well, in answer to the question "What the f--k is an ombudsman doing if he's not paying sufficient attention to Norm Ornstein?," the Times has a new ombudsman since I was quoted on the topic, and she's dedicated a blog post today to airing and agreeing with many of Ornstein's grievances. Though let it be noted Times public editor Margaret Sullivan does think he's too hard on fact checking—“one of the most positive trends in journalism that I can remember.”

Oh, and can someone lend me an electron microscope so I can find a Stradivarius suitable for this?:

It's hard to exaggerate just how popular Mann and Ornstein were with the press before their apostasy. They were quite possibly the two most quotable men in Washington. They were the media cocktail party circuit's most reliable walking talking points.

And now they are virtual pariahs.

"It's awkward. I can no longer be a source in a news story in the Wall Street Journal or the Times or the Post because people now think I've made the case for the Democrats and therefore I'll have to be balanced with a Republican," Mann said.

I don't know about being used as source, but as I've previously noted, their views aren't exactly being ignored by the New York Times and Washington Post. And in May, Ornstein and Mann did a lengthy interview on the Wall Street Jounal's website promoting their book. Pariahs, they are not. To the miniscule extent they should feel alienated, I'd venture that making gross generalizations unsupported by facts in the process of insisting half the country is willfully disregarding the truth—well, that could have something to do with it. But again, there's no reason to cry for Mann and Ornstein:

"We've gotten a tremendous amount of attention, but much of that is due to the Internet and our original piece going viral," Mann said. They were also featured on NPR.

There have been countless requests for speaking engagements. "We're just selling a sh-tload of books," said Mann. "There've been page-one stories in countries around the world."